Eleven Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths

Abstract

We present a catalog of 11 multi-planet systems from Campaigns 1 and 2 of the K2 mission. We report the sizes and orbits of 26 planets split between seven 2-planet systems and four 3-planet systems. These planets stem from a systematic search of the K2 photometry for all dwarf stars observed by K2 in these fields. We precisely characterized the host stars with adaptive optics imaging and analysis of high-resolution optical spectra from Keck/HIRES and medium-resolution spectra from IRTF/SpeX. We confirm two planet candidates by mass detection and validate the remaining 24 candidates to $>99\%$ confidence. Thirteen planets were previously validated or confirmed by other studies and 24 were previously identified as planet candidates. The planets are mostly smaller than Neptune (21/26 planets) as in the Kepler mission and all have short periods ($P<50$ d) due to the duration of the K2 photometry. The host stars are relatively bright (most have $Kp<12.5$ mag) and are amenable to follow-up characterization. For K2-38, we measured precise radial velocities using Keck/HIRES and provide initial estimates of the planet masses. K2-38b is a short-period super-Earth with a radius of $1.55\pm 0.16R_{\oplus }$, a mass of $12.0\pm 2.9M_{\oplus }$, and a high density consistent with an iron-rich composition. The outer planet K2-38c is a lower density sub-Neptune-size planet with a radius of $2.42\pm 0.29R_{\oplus }$ and a mass of $9.9\pm 4.6M_{\oplus }$ that likely has a substantial envelope. This new planet sample demonstrates the capability of K2 to discover numerous planetary systems around bright stars.

29 pages, 20 figures, accepted to the Astrophysical Journal. Updates include K2 names, FPPs, and one more multiplanet system, K2-35

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